Recent advances in the field of semiconductor technology have led to the development of chip carrier packages. A chip carrier package is generally comprised of a planar housing having a semiconductor chip therein. Conductive members, either in the form of metallized pads on the planar housing, or wire-like leads extending from the housing, serve to connect the semiconductor chip to a set of metallized areas on a printed circuit board. In addition to chip carrier packages, other types of electronic components, such as resistors and capacitors, for example, have also been developed for mounting to the metallized areas on the surface of the printed circuit board. The term "surface-mounted component" is commonly used to generically describe any type of electronic component which is designed for mounting to the metallized areas on the surface of a printed circuit board.
Currently, surface-mounted components are mounted on a printed circuit board to form an article, referred to as a "circuit pack," by the following process. First, a controlled amount of a reflowable bonding material (e.g., solder paste) is applied to the metallized areas on the printed circuit board. Next, the surface-mounted component is placed on the printed circuit board so each of the conductive members of the component contacts a separate one of the solder paste-coated, metallized areas. In practice, the solder paste is tacky and serves to adhere the conductive members of the component to the metallized areas. Thereafter, the printed circuit board and the surface-mounted component are heated to reflow the solder paste to create a bond between each conductive member of the component and each metallized area on the printed circuit board.
Generally, the solder paste is selectively deposited (printed) onto the metallized areas on the board by forcing the paste through openings in a foraminous member, typically a stencil, placed proximate with the printed circuit board so as to be spaced a short distance thereabove. The openings in the stencil are arranged in a pattern corresponding to that of the metallized areas on the board to be coated with the solder paste. The process of forcing the paste through the openings in the stencil is accomplished by depositing the paste on the stencil and then sweeping a flexible member (e.g., squeegee or doctor blade) thereacross. Usually, the metallized areas on the printed circuit board exposed through the openings in the stencil can be coated with solder paste after one or two passes of the squeegee blade across the stencil.
An apparatus designed for printing solder paste onto a printed circuit board is disclosed in U.S. Pat. No. 4,622,239 (hereby incorporated by reference), issued on Nov. 11, 1986, in the names of D. Schoenthaler and T. Wojcik and assigned to AT&T Technologies Inc. The Schoenthaler et al. solder paste dispensing apparatus comprises a housing containing a quantity of solder paste. The housing has a bottom opening located between a pair of flexible members (squeegee or doctor blades) which extend from the housing for contacting the stencil. In operation, the solder paste is forced out from the opening in the housing so as to be confined between the flexible members. As the solder paste is expelled from the housing, a relative motion is imparted between the stencil and the housing so the flexible members force the solder paste confined therebetween into the stencil openings.
In practice, both of the flexible members remain in contact with the stencil so the solder paste is confined between the flexible members, and thus the exposure of the expelled solder paste to the atmosphere is reduced. In this way, premature drying of the solder paste is substantially prevented.
When printing extremely viscous solder paste (e.g., .gtoreq.700,000 centapoise) with the Schoenthaler et al. dispenser, problems can arise. As the solder paste is dispensed, the solder paste adjacent to the trailing one of the flexible members will experience a very high shear force. This shear force separates the paste into its constituent components (e.g., solder spheres and a suspending viscous flux). Some of the flux tends to pass underneath the trailing flexible member, forming a thin (0.0005" thick) flux film on the stencil. After repeated printing of the solder paste on the stencil, the solder paste confined between the flexible members becomes substantially depleted of flux. This is because during each pass of the dispensing apparatus across the stencil, the flux film left thereon during the prior pass of the dispenser is pushed ahead and off to the end of the stencil by the leading flexible member.
Since the flux is pushed ahead and off to the end of the stencil, the solder paste confined between the flexible members does not mix therewith and remains depleted of flux. The deposited solder paste, when depleted of flux, tends to remain in the stencil openings instead of adhering to the metallized areas on the printed circuit board. Consequently, the stencil must be removed and cleaned, with the attendant cost in downtime.
In contrast, the problem of clogging is usually not experienced by the solder paste printing apparatus which employs a single squeegee or doctor blade. The solder paste printed by a conventional, single squeegee blade apparatus will also experience high shear forces, causing the solder paste to separate and the deposition of a thin film of flux on the stencil. However, as the conventional solder paste printing apparatus is moved in opposite directions across the stencil, the solder paste leaving the dispensing apparatus is mixed by the single squeegee blade of the dispenser with the flux film previously left on the stencil. In this way, any change in the flux content of the solder paste forced into the stencil openings is small and clogging of the stencil is avoided. However, the paste printed by a conventional single squeegee blade solder paste apparatus is not shielded from exposure to the atmosphere, causing premature drying of the solder paste. This necessitates cleaning of the apparatus, with the attendant downtime associated therewith.
Accordingly, there is a need for a method for printing a highly viscous material, such as solder paste, through openings in a stencil without the clogging of the stencil openings and drying of the material.